Tape constructions for disposable diapers and their manufacture and assembly are described for instance in U.S. Pat. Nos. 4,171,239 to Hirsch et al. and 4,378,800 to Schaar each incorporated herein by reference.
Two different but cooperating tapes are used in disposable diaper tape manufacture. Each can consist of a polyolefin film having a release coating on one surface and a pressure sensitive adhesive on the opposed surface. One is known as a fastening tape and used in practice to close the diaper. The other is a release tape whose function is to protect the adhesive of the fastening tape until used.
They are combined at a "Y" junction with each tape supplying pressure-sensitive adhesive for bonding to the diaper shell. The release tape has its release layer against the adhesive of the fastening tape until use. They are manufactured from pressure-sensitive adhesive tapes on self-wound rolls--each having an adhesive on one side of a carrier film (e.g. polypropylene) and a cured release coating on the other.
The process of manufacture involves in-line folding of each tape, laminating them together, cutting to length and applying the combination to the diaper. Diapers are manufactured in a continuous web which passes through a tape lamination, cutting and application station.
More particularly, the fastening tape is folded over on itself at one end to form a fingerlift or tab, a thickened area to assist the user in grasping the "Y" end of the fastening tape and separate it from the release surface of the release tape. The release tape fold is used to create a strong bond at the edge of the diaper. The adhesive of the release tape and a portion of the adhesive of the fastening tape are bonded to opposed surfaces of a diaper construction. The balance of the fastening tape is secured to the release surface of the release tape.
Various joining and folding processes occur. The joining process involve changes in tape direction including a right angle turn which completes the fold.
If the tape encounters any significant drag on means to guide the tape through a turn, unequal tensions will form and cause the edge to wander. This gives poor alignment of the fold and inconsistent width to the folded edge.
After both folds are made the tapes are laminated together by aligning them prior to bringing them together. This alignment process involves passing the tapes over guide bars which contact the release surfaces of the tapes. Again, differential drag across the guide bars prevents smooth lamination since one tape may be traveling at a different speed relative to the other. Poorly aligned laminated tapes are difficult to guide through the remainder of the process and may have poor appearance.
The release surface of the release tape surface must also provide a fairly high bond to the fastening tape adhesive. This ensures the two tapes will stay together during the cutting and actual application to the diaper shell. It is particularly important while the lamination is being applied to the diaper shell. When the laminated tapes are cut and applied to the bottom of the formed diaper, the tapes are positioned so that the fastening tape/the release tape laminate extends beyond the edge of the formed diaper. A jig is used to guide the free end around the edge of the diaper. All of this occurs while the diaper moves at significant speed. A low bond strength between the fastening tape adhesive and the release tape at this application may allow the lamination to open during the severe turn and lead to a defective product.
In the past the cured release coating has been tin catalyzed. This has resulted in variable release and unwind performance in consequence of post curing (continuation of cure after the initial cure) of the applied release coating in the roll with release properties being somewhat dependent on how long bulk rolls were kept in inventory before moving the rolls into diaper production operations. It is understandably desirable to have available a low friction release coating to allow tape to slide over fixed posts used to change direction in tape alignment and joining operations in the manufacture of the diaper tape. By the same token, the adhesion or release level (force to cause release) must be sufficiently high to provide sufficient unwind tension to keep the release tape in a guide and prevent premature unraveling of the roll. Release is also important to keep the fastening tape/release tape combination together as it progresses through manufacturing operations. High release is also necessary to prevent the fastening tape from "flagging" as the lamination is folded around a diaper and to stay on the release surface of the release tape up to the time of use.
Prior silicone release coating do not provide stable low friction and high adhesion release surfaces. It would be desirable to provide a silicone release surface for the fastening and/or release tape elements of diaper tapes which provides the low friction necessary for assembly of the diaper tape without distortion due to tape stretching, and at the same time to provide sufficiently high adhesion to meet the various needs of disposable diaper release tape manufacture and use.
To provide such a surface for tapes, particularly polypropylene film tapes, is the purpose of the instant invention.